2021-06-05 22:39:01 +08:00
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# DAG
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`hyperf/dag` 是一個輕量級有向無環圖 (**D**irected **A**cyclic **G**raph) 任務編排庫。
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## 場景
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假設我們有一系列任務需要執行。
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- 如果他們之間存在依賴關係，則可以將他們順序執行。
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- 如果他們並不相互依賴，那麼我們可以選擇併發執行，以加快執行速度。
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- 兩者間還存在中間狀態：一部分任務存在依賴關係，而另一些任務又可以併發執行。
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我們可以將第三種複雜的場景抽象成 `DAG` 來解決。
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2021-07-19 13:00:24 +08:00
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## 安裝
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```bash
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composer require hyperf/dag
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```
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2021-06-05 22:39:01 +08:00
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## 示例
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[](https://mermaid-js.github.io/mermaid-live-editor/#/edit/eyJjb2RlIjoic3RhdGVEaWFncmFtLXYyXG4gICAgWypdIC0tPiBBXG4gICAgQSAtLT4gQlxuICAgIEEgLS0-IENcbiAgICBBIC0tPiBEXG4gICAgRCAtLT4gR1xuICAgIEMgLS0-IEdcbiAgICBDIC0tPiBGXG4gICAgQiAtLT4gRlxuICAgIEIgLS0-IEVcbiAgICBCIC0tPiBIXG4gICAgSCAtLT4gSVxuICAgIEUgLS0-IElcbiAgICBGIC0tPiBJXG4gICAgRyAtLT4gSVxuICAgIEkgLS0-IFsqXVxuICAgICAgICAgICAgIiwibWVybWFpZCI6eyJ0aGVtZSI6ImRlZmF1bHQiLCJ0aGVtZVZhcmlhYmxlcyI6eyJiYWNrZ3JvdW5kIjoid2hpdGUiLCJwcmltYXJ5Q29sb3IiOiIjRUNFQ0ZGIiwic2Vjb25kYXJ5Q29sb3IiOiIjZmZmZmRlIiwidGVydGlhcnlDb2xvciI6ImhzbCg4MCwgMTAwJSwgOTYuMjc0NTA5ODAzOSUpIiwicHJpbWFyeUJvcmRlckNvbG9yIjoiaHN
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假設我們有一系列任務，拓撲結構如上圖所示，頂點代表任務，邊緣代表依賴關係。(A 完成後才能完成 B、C、D，B 完成後才能完成 H、E、F...)
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通過 `hyperf/dag` 可以使用如下方式構建 `DAG` 並執行。
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```php
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<?php
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$dag = new \Hyperf\Dag\Dag();
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$a = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "A\n";});
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$b = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "B\n";});
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$c = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "C\n";});
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$d = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "D\n";});
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$e = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "E\n";});
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$f = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "F\n";});
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$g = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "G\n";});
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$h = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "H\n";});
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$i = \Hyperf\Dag\Vertex::make(function() {sleep(1); echo "I\n";});
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$dag->addVertex($a)
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->addVertex($b)
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->addVertex($c)
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->addVertex($d)
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->addVertex($e)
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->addVertex($f)
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->addVertex($g)
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->addVertex($h)
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->addVertex($i)
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->addEdge($a, $b)
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->addEdge($a, $c)
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->addEdge($a, $d)
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->addEdge($b, $h)
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->addEdge($b, $e)
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->addEdge($b, $f)
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->addEdge($c, $f)
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->addEdge($c, $g)
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->addEdge($d, $g)
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->addEdge($h, $i)
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->addEdge($e, $i)
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->addEdge($f, $i)
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->addEdge($g, $i);
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// 需要在協程環境下執行
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$dag->run();
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```
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輸出：
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```php
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// 1s 後
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A
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// 2s 後
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D
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C
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B
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// 3s 後
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G
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F
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E
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H
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// 4s 後
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I
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```
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> DAG 會按照儘可能早的原則調度任務。嘗試將 B 點的耗時調整為 2 秒，會發現 B 和 G 一起完成。
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## 訪問前步結果
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每一個任務可以接收一個數組參數，數組中包含所有前置依賴的結果。`DAG` 執行完畢後，也會返回一個同樣結構的數組，包含每一步的執行結果。
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```php
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<?php
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$dag = new \Hyperf\Dag\Dag();
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$a = \Hyperf\Dag\Vertex::make(function() {return 1;});
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$b = \Hyperf\Dag\Vertex::make(function($results) use ($a) {
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return $results[$a->key] + 1;
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});
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$results = $dag->addVertex($a)->addVertex($b)->addEdge($a, $b)->run();
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assert($results[$a->key] === 1);
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assert($results[$b->key] === 2);
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```
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## 定義一個任務
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在上述文檔中，我們使用了閉包來定義一個任務。格式如下。
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```php
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// Vertex::make 的第二個參數為可選參數，作為 vertex 的 key，也就是結果數組的鍵值。
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\Hyperf\Dag\Vertex::make(function() { return 'hello'; }, "greeting");
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```
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除了使用閉包函數定義任務外，還可以使用實現了 `\Hyperf\Dag\Runner` 接口的類來定義，並通過 `Vertex::of` 將其轉化為一個頂點。
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```php
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class MyJob implements \Hyperf\Dag\Runner {
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public function run($results = []) {
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return 'hello';
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}
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}
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\Hyperf\Dag\Vertex::of(new MyJob(), "greeting");
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```
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`\Hyperf\Dag\Dag` 本身也實現了 `\Hyperf\Dag\Runner` 接口，所以可以嵌套使用。
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```php
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<?php
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// 命名空間已省略
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$a = Vertex::make(function () { return 1;});
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$b = Vertex::make(function () { return 2;});
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$c = Vertex::make(function () { return 3;});
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$nestedDag = new Dag();
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$nestedDag->addVertex($a)->addVertex($b)->addEdge($a, $b);
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$d = Vertex::of($nestedDag);
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$superDag = new Dag();
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$superDag->addVertex($c)->addVertex($d)->addEdge($c, $d);
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$superDag->run();
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```
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## 控制併發數
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`\Hyperf\Dag\Dag` 類提供了 `setConcurrency(int n)` 方法控制最大併發數。默認為 10。
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